The measuring device includes: A mechanically oscillatable unit, which can be secured via a securement to a sensor housing and/or to the container; and a driver/receiver unit, which excites the mechanically oscillatable unit to oscillate, or receives oscillations of the mechanically oscillatable unit, as the case may be.
Endress + Hauser manufactures and sells fill level measuring devices under the mark LIQUIPHANT. These measuring devices include, as the mechanically oscillatable unit, an oscillatory fork of two tines, which are excited to mechanical oscillations via a membrane, or diaphragm, by a piezoelectric element as driver/receiver unit. In such case, the two tines of the fork oscillate with opposite phase relative to one another. If the medium, whose fill level is to be monitored, comes in contact with the oscillatable unit, then this leads to a change in the frequency and/or amplitude of the oscillations. In this way, the reaching of a fill level can be detected. Correspondingly, it is also possible to detect the subceeding, or falling beneath, of a fill level, i.e. when the oscillating fork is initially covered by medium and then the level sinks. The membrane, on which the oscillatable unit is attached and via which it is excited to oscillate, or via which the oscillations are received, as the case may be, is connected with a housing of the device, or with a container wall, via a securement. Additionally usually provided in the measuring device is an amplifying unit, which amplifies and feeds-back the received signals.
Important in the case of oscillatory forks, but also in the case of all other mechanically oscillatable units used in field, or measuring, devices, is the symmetry of the oscillatable unit. Thus, it must be assured in the oscillating fork that both tines oscillate in opposite phase in such a manner that reaction forces, and moments acting on the securement are minimized as much as possible. To this end, it is e.g. required that both tines have equal mass moments of inertia, thus also equal torques. The oscillatable unit is thus symmetrically constructed, when its oscillations do not lead to reaction forces, and moments, on the securement, thus, when there are components of the mechanically oscillatable unit—e.g. the mentioned tines of the fork—whose oscillations are matched to one another, such that the forces and torques on the securement exactly cancel one another, so that the residual reaction forces, and moments, are balanced as much as possible. If an asymmetry exists—thus, if, for example, one tine is heavier than the other—then it is no longer possible to assure that the forces and moments acting on the securement compensate one another. Such compensation is, among other reasons, important for preventing loss of energy through the securement. Energy loss leads to reduction of oscillation amplitude. Additionally, an asymmetry can lead to jumping of the oscillation frequency unexpectedly between two values, a situation which can corrupt the measurement.
A method in the state-of-the-art for measuring symmetry of an oscillatory fork involves determining the oscillation frequency of each tine separately. If there are differences present, greater than a predetermined tolerance range, then e.g. the weight or the stiffness of the tines is adjusted, e.g. reduced. This measuring of the frequencies is very complicated. It occurs in the context of manufacture of the field devices. In the field, it is, above all, scarcely possible to determine the oscillation frequencies of the individual tines.